Analysis of Sound Frequency Three-Phase Inverter Used in Instrumentation and Maintenance of Aircraft

Transcription

1 Analysis of Sound Frequency Three-Phase Inverter Used in Instrumentation and Maintenance of Aircraft Tun Lin Naing, and Naung Cho Wynn Abstract As our nation is developing country, the industrial sector is developing throughout the country. For modern aircraft industries and laboratories, the static inverter technology is mainly used in application and testing of the flight equipments. Furthermore, they are widely used in laboratories of Aerospace Engineering Universities all over the world. By providing this equipment for the aircraft instrumentation, it can be used as laboratory equipment in Myanmar Aerospace Engineering University. This paper analyses the output stage of the three-phase inverter and use analog control to get require switching conditions. As a control circuit, UA 741 operational amplifiers are used to get all required outputs and National Instrument multi-simulation software is use to simulate required result. Careful and deep studies are made upon the theory of switching mode converter and its design. Keywords Modern aircraft, three-phase inverter, UA 741, switching mode converter. M I. INTRODUCTION EN had been trying to fly throughout the history of mankind. The idea of flying vehicles had developed so far but not succeeded till the early years of twenty century. It was the Wright brothers who wrote the history in the year They flew the first ever aircraft on the earth. The aircraft were developed and used in military, transportation, agriculture and etc. The aircraft were made to fly higher and faster for later. Nowadays, the aircraft can fly higher than 100,000 ft and faster than speed of sound. New generation aircraft rely heavily on electrical power because of wide use of electronic flight instrument systems to generate, regulate and distribute electrical power throughout the aircraft. Aircraft electrical power is used to operate aircraft flight instrument, essential systems and passenger services. Essential power needs to be able to continue safe operations for electrical power system components. Electrical power system components contain ac generator, constant speed drive, integrated drive generator, transformer rectifier unit and generator control unit. In aircraft, the electrical system is primary an ac system. Aircraft electrical components operate on many different voltage both ac and dc. However, most of the systems use 115 V ac with 400 Hz, 20 V dc and 26 V ac is also used in aircraft for lighting. DC is also supplied from a battery installation. The battery provides 28 V dc. The aircraft s nickel cadmium battery is the final source of backup power. It is also possible to change the 28 V dc into 115 V, a 400 Hz with the use of a static inverter. II. MODERN ELECTRICAL SYSTEM OF AIRCRAFT Modern aircraft depend upon the proper functioning operation of their electrical systems for safe and satisfactory operation. Electrical systems are required for power plant control, system control, navigations, communications, and flight control, lights, gallery operation and other functions. With many aircrafts, flight operation cannot be conducted safety without certain essential system. It is therefore apparent that the proper maintenance of aircraft requires that the electrical systems be kept in the best possible condition through inspection, testing and exercise of approved maintenance procedures. To attain reliability in the electrical systems, it is essential that great care be exercised in the selection of components and materials and that each part is installed in such a manner that it will not be subjected to damaging on conditions of any kind. A. General Equipments In general, requirements for aircraft electrical systems are established to assume that the systems will perform their functions reliably and effectively. Electrical systems for all aircraft must be adequate for the intended use. Electric power sources, their transmission cables and associated control and protective devices must be able to furnish the require power at the proper voltage to each load circuit essential for the safe operation of aircraft compliance with the foregoing requirement must be substantiated by an electric-load analysis that accounts for the electric loads applied to the electrical system in probable combinations and for probable durations. Electrical systems, when installed, must be free from hazards in themselves, in their methods of operation and their effects on other parts of the aircraft. They must be protected from fuel, oil, water and other detrimental substances and from mechanical damage such as abrasion or physically applied force. The systems must be designed so that the risk of electrical shock to the crew, passengers, and ground personal is reduced to a minimum. Electrical equipment in system must be so designed that in the event of a fire in the engine compartment, during which the 376

2 surface of the fire wall adjacent to the fire is heated to 2000 F for five minute or to a lower temperature substantiated by the applicant, the equipment that is essential to continued safe operation of the aircraft and located behind the fire wall will function satisfactorily and will not create an additional fire hazard. B. Electrical Power System Components Electrical power system components are (3) Preparation of food. Aircraft electrical components operate on many different voltages both AC and DC. However, most of the systems use 115 V AC with 400 Hz and 28 V DC. Some aircraft use 26 V AC for lightning. GEN 1 GEN 3 GEN 2 GCU 1 GCU 3 GCU 2 (1) AC generator (2) Constant speed drive (3) Integrated drive generator (4) Transformer rectifier unit (5) Generator control unit EPCU AC BUSES GEN 1 GEN 2 GEN 3 EXT PWR ESS AC BUS The ac generator consists of three main parts, the permanent magnet generator, the exciter generator and the main generator. DC power, obtained from a transformer rectifier unit connected to the permanent magnet generator output, is fed via the voltage regulator to the exciter field winding in the generator part. The magnetic field of this exciter field winding induces a voltage in a three-phase rotor winding. The purpose of constant speed drive is to take the rotational power from the engine and no matter the engine speed, turn the generator at a constant speed. This is necessary because the generator output must be 400 Hz. Another method of regulating the speed is with the use of an integrated drive generator (ICG). An IDG is a simply a CSD and generator combined into one unit. Transformer rectifier unit (TRU) is used to transformed 115V ac into 28 V dc. A transformer is used to reduce the voltage from 115 V to 28 V. Output voltage of the ac generator is regulated by controlling the amount of power delivered from the regulator to the generator exciter field winding. The regulator senses the system voltage, compares it with a reference voltage and uses the resulting signal to control generator excitation. The protective functions in the generator control system are overvoltage, under voltage, differential protection, over current and under frequency. C. Electrical Power Usage Aircraft electrical power is used to operate: (1) Aircraft flight instruments (2) Essential systems (3) Passenger services Essential power is power that the aircraft needs to be able to continue safe operation. Passenger services power is the power that applied for: (1) Cabin lightning (2) Operation of entertainment systems APU CONTROL BUS TRU 1 ESS TRU TRU 2 ESS DC BUS DC BUSES Fig. 1 Electrical power principle of a typical aircraft BATTERY SUPPLY D. Aircraft Flight Instruments Aircraft flight instruments are needed to measure pressure, temperature, altitude, velocity, rate of flow, and numerous other conditions or parameters affecting the flight and operation of aircraft. These instruments include: (1) RPM measuring instruments (2) Temperature indicators (3) Fuel quality indicators (4) Position indicators RPM measuring instruments consist of dc tachometer and ac tachometer. Temperature indicators include thermocouple temperature indicators and resistance type temperature indicators. Fuel quality indicators for many light aircraft and all large commercial aircraft are either electrically or electronically operated. The electrically operated indicating systems are usually of the variable resistor type. A synchro system position indicator is designed to measure an angular deflection at one point and reproduce this same deflection at a remote point. Synchro systems have been designed to employ both alternating current and direct current for power. One of the early synchro systems developed by the General Electric Company was the sc Selsyn, often used as an indicator on aircraft with dc power systems to show the position of wing flags and landing gear. 377

3 III. 400Hz SYSTEM Most high power inverter applications are for 50/60 Hz applications or large motor drives. While variable frequency motor drives may require fundamental frequencies in excess of 60 Hz, motors are quite predictable loads. Thus, inverter control strategy for this application is very different from an inverter in a power distribution role. However, there are a small number of applications outside of motor drives for high power inverters with higher fundamental output frequencies. As a standard, aircrafts use 400 Hz electrical system. IV. APPLICATION 400 Hz systems find use in applications where space and weight are at a premium. Because of higher fundamental frequency than traditional line frequencies, passive components in a 400 Hz system can be much smaller. For example, transformers will be smaller in a 400 Hz system, because the volt-second product (change in flux) will be smaller due to the shorter period than that for a 60 Hz system. Smaller passive components enable the power systems to be lighter and take up less volume. In addition, a 400 Hz will enable higher indication motor speeds than are possible in 60 Hz systems. The following equation gives the synchronous speed of an induction motor, 120f Speed(rpm)= (1) P Where f is the fundamental line frequency and P is the number of stator poles. It is easily seem that the synchronous speed of the induction motor is directly proportional to the line frequency. V. IMPLICATIONS FOR CONTROL DESIGN While 400 Hz systems may be beneficial from the power system standpoint, it makes the already difficult task of inverter control even more challenging. In a 400 Hz system, the ratio between the fundamental output frequency and the switching frequency is significantly decreased. This makes the goal of achieving high power quality a more difficult task. Because of the higher fundamental output frequency, it would generally be desirable to increase the resonant frequency of the output filter in order to reduce the size of the passive components. However, increasing the output filter resonant frequency make the task of achieving a control bandwidth greater than the resonant frequency even more difficult to achieve with sufficient stability margins due to control loop delay. VI. INVERTERS An inverter is a device, circuit or system that delivers ac power when energized from a source of dc power. State by another way, inversion is reverse function of rectification. For low and medium power outputs, transistorized inverters are suitable but for high power output, SCRs should be used. For low power, self-oscillation transistorized inverters are suitable but for high power outputs, driven inverters are more common than self-oscillating ones. Moreover, for multiphase ac output there is no alternative other than driven inverters. The driven inverters have better frequency stability than self oscillation because a separate master oscillator is used. A. Types of Inverters Static inverters may be classified into one of the following categories, on the basis of the type of ac output. (1) Voltage source inverter (2) Current source inverter (3) Current regulated inverter (hysteresis type) (4) Phase controlled inverter Of these, the phase controlled inverters does not generate on independent ac. It is an only source to feed power from a dc source into an existing ac source. It is in fact a phase controlled rectifier operating in the inversion mode, that is, with the direction of power flow reversed. B. Classification According to the Method of Communication According to the method of communication, the SCR inverters can mainly be categorized in two types; (1) Line commutative inverters (2) Forced commutated inverters C. Classification According to the Number of Phase Inverters can be classified into two types according to the number of phase. They are- (1) Single-phase inverter (2) Three-phase inverter VII. OVERVIEW OF A SOUND FREQUENCY (400HZ) THREE- PHASE INVERTER Fig. 2 Schematic diagram of a sound frequency (400hz) three-phase inverter This inverter is designed to use in laboratories and industries. Only 220V, 50Hz power supply can be available at these places. So it is designed to use with that power supply. And it also designed to operate with 12V battery supply in case of main power failure. 378

4 28V or 115V, 400Hz, three-phase power supply is used in aircraft instrumentation; therefore the inverter is designed to get the desired output. VIII. SPECIFICATION OF THE CONVERTER CIRCUIT The converter circuit contains two major portions, the amplification circuit, the switching circuit and the output filter circuit. The amplification circuit contains the Darlington pair of transistors and the switching circuit is composed of the bridge circuits with transistor switches. The principal requirement of a switching device used in a switch mode type circuit is that it has fast enough switching times for operation at the desires frequency. As a though guide it should be capable of switching completely on or off in less than five percent of the period. Presently, the solid-state power devices most suited for switching operation in the MHz region, are switch mode and RF MOSFETs. MOSFETs have fast switching times as they are majority carrier devices and hence their turn off time is not dependent on the recombination of majority carriers. The most common and faster power MOSFET is an n-channel enhancement type. An IGBT is more reliable than a MOSFET. But the MOSFETs are available easily in Myanmar and are capable of the desired output for this paper. Converters are most useful circuit in various applications such as inverters, motor control circuits and frequency control circuits. Various types of converters are used to produce the desire output for the systems. The name switching mode converter is given because the converter circuit contain transistors which are used as switches. MOSFETs inverters are very cheap and easy to get in the market. IX. IMPLEMENTATION OF THE SPWM BY SINE-RIANGLE COMPARISON The technique of sinusoidal pulse width modulation, which we described for the single-phase inverter, can also be used in three-phase inverter. Fig. 3 Three-phase bridge to produce required output This technique will enable both voltage and waveform shaping within the inverter. We shall describe here the manner of implementation in three-phase bridge inverters. In the case of three-phase bridge, the phase shift between successive AC output being 120. Therefore, to implement SPWM by sine triangle comparison, it is needed to use three reference sine waves, one for each lag for the inverter with a phase shift of 120 between successive sine waves. Fig. 4 shows the block schematic of the scheme used to obtain the timing instants of the six static switching elements of the inverter. It is the method of the combination of the sine wave and triangular carrier wave to generate pulse width modulation wave form. It has provision for independent adjustment of the frequency and the amplitude. The reference voltage connected to the non-inverting input terminal of the comparator. Therefore, the output of the comparator will go high when the reference wave is more positive than the carrier. Fig. 4 Implementation of SPWM using sine triangle comparison in a three-phase inverter The six timing voltage outputs shown in the figure are numbered to correspond to the numbering of the six static switches in the inverter topology, which is also shown in the Fig. 4. The three sine waves are to have a mutual phase difference of 120, but the same amplitude and frequency. The amplitude of modulation ratio m a can be defined as: Vcontrol m a = (2) V Where V control is the peak amplitude of the control signal or reference sine wave and V tri is the peak amplitude of the triangular signal or carrier triangular wave. The amplitude of the carrier triangular wave is kept constant. The frequency modulation m f is defined as: f tri m f = (3) f tri control Where f tri is the frequency of the triangular wave and the f control is the frequency of the sine reference single. 379

5 X. IMPLEMENTATION OF OVERALL CIRCUIT The all simulation results of the circuit can be got by the used of National Instrument Circuit Simulation Software. Fig. 6 shows the Pulse Width Modulation signal that can be controlled the MOSFETs and Fig. 7 shows the output simulation of the inverter before any filter is consisted. Fig. 8 shows the output waveforms for the three phase case after the filters are consisted. Fig. 5 Overall circuit implementation XI. CONCLUSION This paper has focus on the development and analysis of the 400 Hz, three phase inverter for testing of aircraft instrument in the laboratories and aircraft industries. Electrical power in aircraft is needed to be able to continuous safe operation of electrical power system components such as aircraft flight instruments, essential systems such as ac generator, constant speed drive, integrated drive generator, transformer rectifier unit and generator control unit and passenger services. The control circuit of the inverter is made with operational amplifiers and the switching circuit is use IRF Z44 MOSFETs to generate required output. This paper can give the good knowledge to design the switching mode converter for the inverter. The switching mode converter is investigated and the results that are developed from this research will give the good foundation for the one who want to modify and research in this field with more and more advance technologies in future. Fig. 6 Simulation of the pulse width modulation waveform Fig. 7 Simulation result of the output waveform for one phase ACKNOWLEDGMENT The authors would like to thank for her sincere gratitude to Daw Than Than Win, Professor and Head, department of electrical power engineering, Yangon Technological University and Dr. Ni Ni Win, Lecturer, department of electrical power engineering, Mandalay Technological University for their Patient guidance, criticism, invaluable assistance and encouragement. REFERENCES [1] Dorf. Richard C, Electrical engineering handbook, 2000, by CRC press. [2] Eismin. Thomas K, Aircraft electricity and electronics, Fifth Edition, McGraw-Hill, [3] Kerr. J.R, Maintenance manual of computer controlled UPS and circuit idea. [4] Locher. Ralph, Introduction to power MOSFETs and their applications, Fairchild semiconductor, [5] Skvarenina. Timothyl, The power electronics handbook, Industrial electronic series, 2002, by CRC press. [6] Boeing Co.Ltd, Avionics Training Manual for Fokker F28. Fig. 8 Output voltage waveform for three phase 380